In currently widespread nuclear reactor power plants, nuclear fission in a nuclear reactor is accompanied by generation of a considerable amount of radioactive by-products. The main radioactive substances among the radioactive by-products are fission products and active elements including extremely dangerous radioactive isotopes such as radioactive iodine, radioactive cesium, radioactive strontium, and radioactive cerium. Since radioactive iodine among these radioactive substances turns into a gas at 184° C., there is a risk that the radioactive iodine is extremely liable to be discharged at the time of inspection or exchange of nuclear fuel and furthermore by an unforeseen event such as an accident during handling nuclear fuel or a reactor excursion accident. The major radioactive iodine isotopes to be taken into account at the time of discharge are iodine 129 having a long half-life (half-life: 1.57×107 years) and iodine 131 having a short half-life (half-life: 8.05 days). Here, ordinary iodine that does not exhibit radioactivity is an essential trace element in the human body, is collected in the thyroid gland near the throat, and becomes a component of a growth hormone. Therefore, when a human takes in radioactive iodine through breathing or water/foods, the radioactive iodine is collected in the thyroid gland in the same way as in the case of ordinary iodine and increases internal exposure to radioactivity, and accordingly, a particularly strict measure for reducing the amount of radioactivity to be discharged must be implemented with regard to radioactive iodine.
Moreover, radioactive cesium has a melting point of 28.4° C., is one of metals that become liquid at around a normal temperature, and is a metal that is extremely liable to be discharged as well as radioactive iodine. The major radioactive cesium isotopes to be taken into account at the time of discharge are cesium 134 having a relatively short half-life (half-life: 2 years) and cesium 137 having a long half-life (half-life: 30 years). Among the major radioactive cesium isotopes, cesium 137 in particular not only has a long half-life but also emits high-energy radiation, and has a property that water solubility is high because the radioactive cesium is an alkaline metal. Furthermore, radioactive cesium is easily absorbed in the human body through breathing and also through skin and is uniformly dispersed in the whole body, and therefore a health hazard to humans when the radioactive cesium is discharged becomes serious.
Thus, when radioactive cesium is accidentally discharged due to an unforeseen event or the like from nuclear reactors in operation all over the world, there are concerns that the radioactive cesium causes not only radioactive contamination to workers at nuclear reactors or neighborhood residents but also radioactive contamination over a wider range to humans and animals through foods or water contaminated by the radioactive cesium carried by air. The danger with regard to the radioactive contamination has already been proven undoubtedly by the accident in Chernobyl nuclear power plant.
To such a situation, a cleaning processing system, a physical/chemical processing system by solid adsorbent filling using fibrous activated carbon or the like (see Patent Literatures 1 and 2), processing by an ion exchange material (see Patent Literature 3), and so on have been studied as a method for processing radioactive iodine generated in a nuclear reactor.
However, any of the above methods has problems as will be described below, and the development of a method for removing radioactive iodine in which these problems are solved is desired. First of all, an alkaline cleaning method or the like exists as a cleaning processing system that is practically used, however there are lots of problems in terms of quantity and safety to apply processing by the cleaning processing system with a liquid adsorbent and store the processed liquid as it is for a long period of time. Moreover, in the physical/chemical processing system by solid adsorbent filling, captured radioactive iodine is always facing the possibility of being replaced with other gases, and moreover the processing system has a problem that an adsorbed matter is liable to be discharged when the temperature increases. Furthermore, in the processing system by an ion exchange material, the heat resistant temperature of the ion exchange material is up to about 100° C. and there is a problem that the ion exchange material cannot exhibit sufficient performance at a temperature higher than the heat resistant temperature.
On the other hand, as a method for removal processing of radioactive cesium generated by nuclear fission in a nuclear reactor, an adsorption method with an inorganic ion exchanger or a selective ion exchange resin, a coprecipitation method by using a heavy metal and a soluble ferrocyanide or ferrocyanide salt together, a chemical processing method with a cesium precipitation reagent, and so on are known (see, for example, Patent Literature 4).
However, in any of the above-described processing methods, large scale facilities such as a circulation pump, a cleaning tank, and furthermore a filling tank containing various adsorbents are necessary, and in addition, a large amount of energy to operate these facilities is needed. Moreover, when supply of the power source is suspended as in the accident occurred at the Fukushima No. 1 nuclear power plant in Japan on Mar. 11, 2011, these facilities cannot be operated and, in such a case, the degree of contamination risk by radioactive cesium increases. And particularly in the case where the supply of the power source is suspended, applying a method for removing radioactive cesium diffused into peripheral areas by a reactor excursion accident falls into an extremely difficult situation, and it is concerned that a situation in which radioactive contamination expands may occur. Accordingly, there is an urgent need to develop a technology for removing radioactive cesium that is applicable even when the situation in which the supply of the power source is suspended occurs, and when such a technology for removing radioactive cesium is developed, the method is extremely useful.